There are still approx. 2,000 exchanges that BT need to upgrade from 20CN to 21CN - that work is continuing - though these exchanges only cover a relatively small amount of the UK population. Its all funded by BT. They have done the easy exchanges, and are now left with the smaller or more demanding/challenging ones - for instance, where they don't have space to install new 21CN kit prior to removing the old 20CN stuff.
However, FTTC is/has been being rolled out to many 20CN areas - often funded by BDUK - in these cases the FTTC backbone that feeds the FTTC cabinets bypasses local exchanges and goes to a larger 21CN headend/exchange.